Copper foil is supplied to PCB manufacturers in the form of laminates. These laminates are formed by bonding the copper foil to a polymeric insulating resin. The copper foil portion of the laminate is etched to form the conductive paths of the PCB using processes known to those skilled in the art. The etched copper conductive paths provide electrical connection between various portions of an electronic device.
The copper foil qualities that enable improved performance and reliability are lower controlled profile (roughness), superior ductility, high tensile strength, and thermal stability. The prior art suggests methods of making copper foils having one or more of these attributes, but not all of them simultaneously.
All of these attributes are essential for providing copper foils that are fully acceptable to the PCB industry. For example, foils having profiles that are too high result in laminates exhibiting glass fiber breakage, measles and embedded copper. Foils having profiles that are too low result in laminates with insufficient bond strength. Foils with low ductilities crack when temperature stressed. Thermally unstable foils result in laminates that warp and twist when copper recrystallizes and softens during heating. Foils with low tensile strengths wrinkle during handling. The foils of the present invention overcome all of these problems.
The trend in the PCB industry is to use thinner laminates made with thinner foils and less resin. These laminates are often processed at higher temperatures than heretofore. Additionally, the industry is being required to hold ever tighter tolerances. To meet these challenges the industry is desirous of having thinner foils with low profile and good bonding characteristics. These foils must have a minimum of about 4% elongation at 180.degree. C. for ductility and must be thermally stable. The foils must have tensile strengths high enough to resist damage during handling.
Copper foil producers are desirous of having a process for the manufacture of these foils that is highly tolerant of impurities that are routinely present in copper electrolytes. The difficulties and expenses associated with providing essentially pure electrolytes render processes requiring such electrolytes non-competitive.
Lakshmanan et al, "The Effect of Chloride Ion in the Electrowinning of Copper", Journal of Applied Electrochemistry 7 (1977) 81-90, discloses that the effect of chloride ion concentration on copper electrodeposition is dependent on the operating current density. At lower current density values the ridge type growth structure orientation is favored for additive-free electrolytes. At high current density values pyramidal growth orientation is favored for additive-free electrolytes. The addition of chloride ion to the 10 ppm level lowers the overvoltage and thus promotes ridge type oriented deposits. As the current density is increased to 40 amps per square foot [0.043 A/cm.sup.2 ], the pyramidal growth structure is again favored. The article indicates that the current densities that were tested ranged from 15 to 40 amps per square foot [0.016 to 0.043 A/cm.sup.2 ].
Anderson et al, "Tensile Properties of Acid Copper Electrodeposits", Journal of Applied Electrochemistry, 15 (1985) 631-637, discloses that the chloride ion concentration in an acid copper plating bath influences the ultimate tensile strength and elongation of the foil produced therefrom. The article indicates that at the current densities tested, acid copper plating baths require the presence of chloride ions to provide a ductile copper deposit. The current densities reported in the article ranged from 20 to 50 mA/cm.sup.2 [0.02 to 0.05 , A/cm.sup.2 ]. Chloride ion concentrations in the range of 0 to 100 ppm are reported.
U.S. Pat. No. 2,475,974 discloses a process for making copper deposits having tensile strengths of about 60,000 to about 73,000 psi and elongations of 6% to 9% using a copper plating solution containing triethanolamine.
U.S. Pat. No. 2,482,354 discloses a process for making copper deposits having tensile strengths of about 65,000 to about 90,000 psi and elongations of 8% to 12% using a copper plating solution containing tri-isopropanolamine.
U.S. Pat. No. 4,956,053 discloses an electrodeposition process for making copper foil using an additive-free electrolyte solution. The reference indicates that the "impurities, especially organics, sulfides and chlorides are kept in concentrations of less than about 5 parts per million and preferably less than about 1 part per million and most preferably at the limits of detection, in the range of parts per billion" (col. 8, lines 8-12). The disclosed process includes a filtration and replenishment system to maintain the electrolyte impurity and particulate free.
U.S. Pat. No. 5,181,770 discloses an electrodeposition process for making copper foil using an electrolyte solution having a chloride ion concentration of either 0.5-2.5 ppm or 10-50 ppm. The reference indicates that all organic and inorganic additives as well as impurities are excluded from the electrolyte.
WO 91/19024 discloses electrodeposited copper foils having an elongation measured at 180.degree. C. in excess of about 5.5%, an ultimate tensile strength measured at 23.degree. C. in excess of about 60,000 psi, and a matte-side R.sub.tm in the range of about 4.5 to about 18 .mu.m. This reference also discloses a process for making electrodeposited copper foil which comprises: preparing a copper deposition bath comprising water, copper ions and sulfate ions, said bath containing less than about 20 ppm chloride ions; and applying electric current to said bath to electrodeposit copper from said bath using a current density in the range of about 200 to about 3000 amps per square foot [0.22-3.23 A/cm.sup.2 ].